&reftitle.constants; &extension.constants; SID (string) Constant containing either the session name and session ID in the form of "name=ID" or empty string if session ID was set in an appropriate session cookie. This is the same id as the one returned by session_id. PHP_SESSION_DISABLED (int) Return value of session_status if sessions are disabled. PHP_SESSION_NONE (int) Return value of session_status if sessions are enabled, but no session exists. PHP_SESSION_ACTIVE (int) Return value of session_status if sessions are enabled, and a session exists.